Without limiting the scope of the disclosure, its background is described in connection with 3-D printing/additive manufacturing.
Multi-material and composite additive manufacturing (AM): Objet's (Rehovot, Israel, now merged with Stratasys) Polyjet™ technology can print structures from two dielectric photopolymers. Multi-material stereolithography using multiple vats of liquid (dielectric) photopolymer has been demonstrated [Wicker et al., 2009], and multi-material FDM has been explored [Espalin, 2012]. Ceramic and metal composites made with FDM have been described by several researchers [Kumar and Kruth, 2010; Vaidyanathan et al., 1999; Onagoruwa et al., 2001; McNulty et al., 1998; Agarwala et al., 1996] and FDM-produced injection molding dies using metallic composites were made [Masood and Song, 2004] and characterized for thermal conductivity [Nikzad et al., 2011].
Electromechanical structures by AM: FDM of ABS and low-melting point alloys such as Bi58Sn42 has been used to make simple multilayer structures having a dielectric structural component and an electrical conductor [Mireles et al., 2012]. However, this approach is limited by the relatively high electrical resistance of solders (Bi58Sn42 solder has ˜22 times higher resistivity than annealed Cu); maintaining the integrity of solder melting at 138° C. while adjacent to polymer deposited at a higher temperature; the inability to use solder to make magnetic elements for electromagnetic actuators; mechanical weakness; brittleness common in Bi-based solders; significant electromigration risk; mutual adhesion of molten solder to polymer; and throughput (polymer and metal dispensed from separate nozzles). Others have demonstrated simple electromechanical/electronic devices fabricated by AM including relays, timing circuits using integrated circuits added manually, and flashlights [Periard et al., 2007; Malone and Lipson, 2007; Alonso et al., 2009; Malone and Lipson, 2008]. For example, a solenoid was fabricated using solder for coils, silicone for dielectric, and iron powder in grease for a core [Alonso et al., 2009]. While useful as a demonstration, the process was cumbersome and not fully-automated. Stratasys (Eden Prairie, Minn.) and Optomec (Albuquerque, N. Mex.) have demonstrated fabricating structures in rigid polymer using FDM and depositing traces of silver nanoparticle ink using aerosol jetting on the exterior surfaces of the structure [O'Reilly and Leal, 2010]. Trace resistivity can be as low as 1×10-5 ohm-cm, but part surface roughness, applicability to accessible surfaces only, and the need to sinter the ink remain challenges. Similar work has been done by researchers using stereolithography and micro-dispensing pumps [Lopes et al., 2012]. In both these efforts, traces are necessarily confined to external surfaces unless channels are manually filled by pumping [DeNava et al., 2008]; therefore the circuitry, like that produced by O'Reilly and Leal, is not truly 3-D and solenoid-type coils seem impossible. Moreover, these processes are not integrated or fully-automated. Others have postulated the use of curved layers to produce integrated electromechanical structures using FDM, insisting incorrectly that circuits cannot be produced using planar processes due to inter-layer connectivity issues [Diegel et al., 2011]. Curved layers introduce many difficulties and in any case do not truly obviate the need for a solution to interlayer connectivity.
More recently, Voxel8 Inc. (Somerville, Mass.) has developed a multi-material 3-D printer using FDM to deposit a thermoplastic polymer as dielectric and structural element, and an extrusion head to deposit between the layers of thermoplastic a rapidly-setting silver-based conductive ink to form interconnects. However, the Voxel8 process requires a separate step to deposit the conductor, and the ink is approximately 30 times higher in resistivity than metallic copper, has unknown maximum current density, and is extremely costly (indeed, over 1500 times as costly as copper wire for a “wire” of the same total resistance).
Wire embedding AM: While most prior efforts to integrate conductive and dielectric materials in AM assume that the conductive material cannot be a solid metal, a student project called “SpoolHead” investigated the use of FDM and metal wire to make 3-D circuits [Bayless et al., 2010], inspired by an adhesive-coated wire-based AM method [Lipsker, 2000]. Earlier work [e.g., Rabinovich, 1996] explored generating 3-D structures using laser welding of flat-sided wire. The SpoolHead project aimed to deposit thermoplastic using FDM, then interrupt the process and lay down wire while attempting to secure it to the polymer by remelting. Later, a process developed at the University of Texas at El Paso began to develop an approach [Aguilera et al., 2013] similar to SpoolHead, but in which wire is pushed just below the surface of a printed layer using either ultrasonic vibration or Joule heating to reflow the thermoplastic matrix material, allowing wire embedding. Junctions between wires or between wires and added components are created by laser welding. Due to the complex and costly equipment required and the relatively low processing speed, the process is not economical for producing most electromechanical or electronic devices, especially in significant volume. Moreover, the embedding process requires that the matrix be reflowable.
Elastomer AM: Additive manufacturing with elastomer materials is currently available. Polyjet can print with elastomeric photopolymer, and 3D Systems' (Rock Hill, S.C.) selective laser sintering process can work with powdered elastomer. Both techniques produce rather fragile parts, and neither is capable of selectively incorporating conductive materials. Elastomers have been cast and combined with other materials using a subtractive/additive process [Cutkosky and Kim, 2009]. Of most relevance, FDM of thermoplastic elastomers was demonstrated at Virginia Tech [Elkins et al., 1997] by changing the design of a standard FDM printhead to reduce the risk of filament buckling and to optimize filament feed rollers. Also, Stratasys commercialized for some time an elastomer FDM material branded as E20.
Molded Interconnect Device: Molded interconnect device (MID) is a device produced via injection molding of thermoplastic and having circuitry integrated into the device. The process is limited to locating circuit elements on the surface of the device; they cannot be located internally, so it would, for example, be impossible to produce a multi-layer, 3-D coil. Moreover, MID conductors tend to be thin and not capable of carrying higher currents.